1. Field of the Invention
This invention relates to a magnetic memory device utilizing a magnetic disc, a magnetic drum or a magnetic tape for use as a magnetic recording medium, and more particularly to a breathing device for a closed housing of a magnetic memory device.
2. Description of the Prior Art
In a magnetic memory device of the type referred to above, information is written into or read out from the magnetic recording medium through a magnetic head which is separated from the medium by a thin air gap the width of which is on the order of submicrons. For this reason, unless the interior of the housing containing the magnetic medium is maintained in a clean state free of any appreciable amount of dust, the recording medium will be contaminated or damaged thus degrading or destroying the recorded information. Also, since the recording medium is usually made of such metallic magnetic materials as Ni-Co-P alloys, unless the space in the housing is maintained at a low moisture condition, the magnetic materials will be oxidized to degrade the magnetic characteristics of the magnetic recording medium.
Furthermore, in a contact-start-stop recording system commonly utilized in a magnetic disc device, the magnetic head is in contact with the recording medium when the rotating disc has stopped or when its rotation is at a speed lower than a prescribed number of revolutions. If the air in the housing is maintained at a high humidity, the moisture contained in the air between the magnetic head and the magnetic recording medium will condense to form dew which creates a force of attraction between the head and the recording medium thus causing them to firmly adhere to each other. However, during the writing and reading out of the information, since an air layer, although extremely thin, exists between the magnetic head and the surface of the magnetic recording medium, the adhesion described above will not occur.
Although it is possible to construct the housing containing the magnetic recording head and the recording medium so that it is perfectly air tight, when designing the housing it is necessary to take into consideration the thermal expansion caused by the temperature rise within the housing caused by the running magnetic recording medium. In some cases, the difference between the minimum air temperature in the housing which occurs at non-run times and the maximum air temperature during running times exceeds 50.degree. C. or more. Also the pressure difference between the inside and outside space of the housing, which causes thermal expansion and contraction of the air in the housing is larger than 0.1 atmospheres. In order to construct the sealed housing so that it has sufficient strength to withstand such a pressure difference, it is necessary to use a strong cover or an air tight seal thus complicating the construction and increasing the costs.
For this reason, according to a recent design, the housing is constructed as an ordinary closed housing and an air filter is provided between inside and outside thereof. Furthermore, a desiccator is installed in the housing to remove the moisture which is admitted into the housing mainly through the breather filter.
With this construction, however, it is not only necessary to add an air filter but also to use a substantial quantity of the desiccating agent. In other words, the outside surface of the air filter will be clogged by the dust deposited thereon when the air port "breathes" in accordance with variations in the housing temperature. To prevent such clogging over a long period of time, it is necessary to increase the area of the air filter. For example, when a commercially available air filter made of paper is used to accomodate a recording medium housing having a volume of 5l, it is necessary to make its area about 10 cm.sup.2. Use of a filter having such a large area decreases the flow resistance at the suction port which substantially decreases the resistance to the air flowing into the chamber. Accordingly, when about 300 g of silica-gel is used as the desiccating agent it loses its moisture removing capability in only 3000 to 4000 hours, thus requiring frequent renewal of the desiccating agent. Thus, even though the frequent air suction and exhaust decreases the temperature difference and eliminates the pressure difference, diffusion occurs continuously due to the difference in the vapor pressures between the inside and outside of the housing. Therefore, the admission of moisture into the housing is inevitable. The amount of the moisture admitted into the housing from the diffusion described above is about 5 times that of the moisture suctioned into the housing due to the air pressure difference. Consequently, it becomes apparent that it is necessary to use a large amount of desiccating agent for removing all moisture.